The horizontal deflection stages of monitors and television screens provide for generating a substantially sawtooth current in a deflection yoke, which in turn produces the magnetic field required for sweeping the electron beam of the cathode-ray tube.
For this purpose, unlike vertical deflection stages, currently used horizontal deflection stages employ switch circuits, due to the values of the physical quantities involved. FIG. 1 indicates a typical known deflection stage comprising an NPN transistor 2 defining the switching element, and having the emitter grounded, the collector connected to one terminal of primary winding 3 of a line transformer (shown only partially), and the base defining the control terminal to which is supplied a rectangular control signal V.sub.1. The other terminal of winding 3 is connected to the positive supply, while the collector of transistor 2 is connected to the cathode of a recirculating or flyback diode 4 (the anode of which is grounded), to one terminal of a so-called "flyback" capacitor 5 (the other terminal of which is grounded), and to one terminal of a deflection yoke 6 represented in FIG. 1 by a coil. The other terminal of yoke 6 is connected to one terminal of a control capacitor 7, the other terminal of which is grounded.
FIGS. 2a-2d illustrate the voltage potentials at various nodes over time. Assuming the components are ideal, when transistor 2 is turned off by control signal V.sub.1 switching to low, at instant t.sub.0 as shown in FIG. 2a, capacitor 5 and coil 6 (as well as capacitor 7, the high value of which, however, makes it negligible as compared with capacitor 5) define a freely oscillating resonant circuit, so that the voltage in capacitor 5 (equal to collector-emitter voltage drop V.sub.cet) assumes a sinusoidal waveform as shown in FIG. 2b. The yoke current i.sub.g (which was maximum at instant to) decreases sinusoidally to minimum, so that the electron beam returns to the starting point, shown in FIG. 2c. At instant t.sub.1, the capacitor voltage returns to zero, thus turning on diode 4, which maintains a zero voltage, thus preventing further oscillation of circuit 5, 6. In this phase, current i.sub.g in coil 6 is recirculated via diode 4, and presents a waveform (FIG. 2c) depending on the voltage at the coil terminals and, therefore, on the voltage at the terminals of capacitor 7, which in this phase is parallel to coil 6. More specifically, if the capacitor voltage were constant, the waveform of the current would be linear, increasing as of instant t.sub.1 so as to produce an ideal sawtooth waveform (at least over the useful, i.e. rising, deflection portion, as shown by the dotted line in FIG. 2c). In actual practice, however, by virtue of the cathode-ray tube screen being roughly flat, as opposed to semispherical (with a constant radius), the rising portion of the deflection current waveform necessarily assumes the form of an S, as shown by the continuous line in FIG. 2c. This is due to capacitor 7, which is charged during conduction of diode 4, and discharged in the following interval, thus resulting in a parabolic voltage at the yoke terminals and the S-shaped current waveform shown in FIG. 2c.
Theoretically, the above situation should continue up to instant t.sub.3, at which point, the current in the yoke is zeroed, thus turning off diode 4 and turning on transistor 2 for increasing the yoke current over the positive portion. In actual fact, however, due to the time lapse involved in turning off diode 4 and turning on transistor 2, the latter is turned on in advance so that a further control pulse is produced at instant t.sub.2. Consequently, the transistor is turned on and saturated, as shown, thus maintaining a substantially zero V.sub.cet voltage, so as to discharge capacitor 7 and increase the yoke current as shown by the S-shaped curve.
The above operating mode is based on the assumption (which is actually false) that, when conducting, diode 4 and transistor 2 are capable of maintaining a zero V.sub.ce voltage (zero direct current voltage of diode 4 and saturation voltage of transistor 2), in which case, said voltage would present the theoretical V.sub.cet curve shown in FIG. 2b and described so far. In actual fact, however, voltage V.sub.ce presents the curve shown in FIG. 2d, which in interval t.sub.1 -t.sub.2, presents a negative portion due to the direct current voltage of diode 4; and, in the interval t.sub.3 -t.sub.0, a rising portion of other than zero due to the saturation voltage of transistor 2. As such, the actual curve of yoke current i.sub.g differs from that shown in FIG. 2c, thus resulting in serious, and highly visible, distortion of the picture, particularly in the first part of the screen. This is a known, prior art circuit.
Existing solutions devised to overcome the above drawback provide for an additional coil series connected to deflection coil 6, and the inductance of which varies according to the direction of the current, so as to compensate, in particular, the direct current voltage drop in diode 4 when conducting.
Though relatively cheap, the above solution provides for no more than a partial solution to the problem, in that it fails to compensate for distortion caused by the transistor, and even that of the diode is compensated by a fixed amount, thus resulting in only approximate correction. A further drawback of the above known solution is that the inductance of the additional coil is highly dependent on the characteristics of the transistor and the diode, so that the cost advantage is offset by requiring painstaking adjustment for achieving a high degree of performance.